Concept for a crystal structure model

[jamesrhester]

This describes the concept used for modelling the vast bulk of crystalline materials. Please review and comment.

[rartino]

Should this be "crystal structure model" or simply "crystal structure"?

[rartino]

Based on @jamesrhester's suggestion we can take the discussion about disorder here from #16, and once settled also update both PRs.

Should the crystal structure concept allow site disorder, i.e., sites with statistical occupation, assemblies, attached atoms? (See #16 for more details.) Does a site-disordered compound "have a crystal structure"?

I think one of our guiding principles is "if in doubt, follow cif/iUCr". That principle is also helpful to connect this ontology to the cif ontology. On the practical side, cif files allow expressing unit cells with these three types of disorder (which is one reason we adopted these types of disorder also into OPTIMADE "structures"). On the other hand, the two (real-space and reciprocal space) iUCr dictionary definitions of crystal structure are perhaps not crystal clear (pun intended) on if (what type of) disorder is accounted for:

• In the real-space definition it seems to come down to what one reads into "on average" in the formulation "on average, a long-range ordered arrangement". Is "on average" only meant to account for the "obvious" displacements of atoms due to vibrational disorder, or also various forms of site disorder? Reading it very strictly, I think both should be included.
• For the reciprocal lattice definition, it comes down to the interpretation of "essentially a sharp diffraction pattern", which IMO holds for site disordered compounds.

So, my conclusion is "yes" to that a "crystal structure" concept should allow site disorder, and arguably more obviously vibrational disorder, but not configurational disorder.

As a final remark: my intent when writing up 'unit cell' was that all non-periodic disorder should be collected in a defect concept, accommodating point, line, surface, and volume defects.

[CasperWA]

We should add to the "discussion" section the discussion of the 2/3-23 meeting concerning EMMO icons, the use of the word "model" and whether this should be under the "Data" perspective instead.